Semiconductor memory tester

ABSTRACT

In a semiconductor memory tester in which a test pattern from a pattern generator is applied to a plurality of memory devices installed on a test head and their outputs are each logically compared by a logical comparator with an expected value for each pin, there are provided a plurality of OR circuits each of which obtains the OR of the results of logical comparisons corresponding to predetermined plural pins of each memory device. A plurality of multiplexers, each of which is supplied with the results of logical comparisons for corresponding output pins of the plurality of memory devices and a different one of the plurality of OR outputs, are provided for each group of corresponding output pins and each selectively output any one of the plurality of results of logical comparisons and the OR input thereto in accordance with a select signal. The outputs of these multiplexers are distributed to and stored in a plurality of fail memories.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor memory tester which has an arrangement in which a test pattern is applied to each of a plurality of memory devices under test mounted on a test head, the output from each memory device is subjected to a logical comparison with an expected value for each pin and the result of logical comparison is stored in a fail memory.

FIG. 1 shows a portion of a semiconductor memory tester of this kind. Memory devices under test A1 and A2 are mounted on a first test head 11A and memory devices B1 and B2 are mounted on a second test head 11B. A test pattern, i.e. data and an address, from a pattern generator 15 is applied to the memory devices A1, A2, B1 and B2, in which the data is written at the address and from which the thus written data is read out into logical comparators 16₁, 16₂, 17₁ and 17₂ for comparison with expected values for each pin. The compared results are output from the comparators 16₁, 16₂, 17₁ and 17₂.

Conventionally, these results of comparison are stored in fail memories in the following manner: as depicted in FIG. 2 in which the results of logical comparison for only first pins of the memory devices A1, A2, B1 and B2 are shown, compared outputs A1₁ and A2₁ for the first pins of the memory devices A1 and A2 on the first test head 11A are both applied to multiplexers 18₁ and 182. The multiplexer 181 responds to a select signal S₁ to select and output one of the two inputs thereto, and the multiplexer 18₂ similarly responds to a select signal S₂ to select and output one of the two inputs thereto. Likewise, the results of comparison B1₁ and B2₁ for the first pins of the memory devices B1 and B2 on the second test head 11B are both provided to multiplexers 21₁ and 21₂. The multiplexer 21₁ responds to a select signal S₃ to select and output one of the two inputs thereto, and the multiplexer 21₂ responds to a select signal S₄ to select and output one of the two inputs thereto. The outputs of the multiplexers 18₁ and 21₁ are input into a multiplexer 23₁, which responds to a select signal S₅ to select and output one of the inputs. Similarly, the outputs of the multiplexers 18₂ and 21₂ are input into a multiplexer 23₂, which responds to a select signal S₆ to select and output one of the inputs. The outputs of the multiplexers 23₁ and 23₂ are stored in fail memories 25₁ and 25₂, respectively.

As a result of this, for example, when the select signals S₁ and S₅ are both low, the comparison result Al₁ is stored in the fail memory 25₁, and when the select signals S₂ and S₆ are high and low, respectively, the comparison result B2₁ is stored in the fail memory 25₂. Also for handling results of logical comparisons of the memory devices A1, A2, B1 and B2 for other pins, there are provided multiplexers similar to those 18₁, 18₂, 21₁, 21₂, 23₁ and 23₂ though not shown, and the fail memories 25₁ and 25₂ are adapted to store at respective addresses the results of logical comparisons corresponding to respective pins.

In the case where only one of the test heads 11A and 11B, for example, 11A, is used, the results of logical comparisons for the two memory devices A1 and A2 loaded on the test head 11A can concurrently be stored in the fail memories 25₁ and 25₂. However, when the both test heads 11A and 11B are used, the following problems exist:

(a) Since the multiplexers 23₁ and 23₂ each select either one of the comparison results from the test heads 11A and 11B, all the comparison results for the memory devices A1, A2 and B1, B2 carried by the test heads 11A and 11B, respectively, cannot be output concurrently. In other words, the comparison result for only one of the memory devices, for example, A1 loaded on the test head 11A and the comparison result for only one of the memory devices, for example, B1 on the test head 11B are stored in the fail memories 25₁ and 25₂.

(b) If the comparison results for both of the memory devices, for instance, A1 and A2 on the one test head 11A are simultaneously stored in the fail memories 25₁ and 25₂, the comparison results for the both memory devices B1 and B2 on the other test head 11B cannot be stored in the fail memories 25₁ and 25₂.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a semiconductor memory tester in which the results of tests of a plurality of semiconductor memory devices under test on a desired one of a plurality of test heads can selectively stored in a plurality of fail memories as in the prior art and in which, if necessary, the results of "good/no good" decision on all the memory devices under test can be stored in a desired fail memory for each test pattern.

According to the present invention, the results of logical comparisons for a predetermined plurality of pins of each of N (where N is an integer equal to or greater than 2) memory devices under test on a first test head are ORed by a first OR circuit, and the results of logical comparisons for a predetermined plurality of pins of each of N memory devices under test on a second test head are ORed by a second OR circuit. A desired one of N+1 outputs including the results of logical comparisons for the corresponding pins of the N memory devices on the first test head and the output of the first OR circuit, corresponding to one of the N memory devices, is selected by a first multiplexer. Similarly, a desired one of N+1 outputs including the results of logical comparisons for the corresponding pins of the N memory devices on the second test head and the output of the second OR circuit, corresponding to one of the N memory devices, is selected by a second multiplexer. One of the outputs of the first and second multiplexers for each of the corresponding pins is selected by one of third multiplexers, and the outputs of these third multiplexers are stored in a plurality of fail memories.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a portion of a conventional memory tester;

FIG. 2 is a block diagram showing multiplexers connected to test heads in the conventional memory tester;

FIG. 3 is a block diagram illustrating a portion of a memory tester embodying the present invention;

FIGS. 4, 4A, and 4B are block diagrams showing the entire multiplexer circuitry used in the memory tester of the present invention;

FIG. 5 is a circuit diagram showing in detail a portion of the multiplexer circuitry depicted in FIG. 4;

FIG. 6 is a circuit diagram showing in detail another portion of the multiplexer circuitry depicted in FIG. 4; and

FIG. 7 is a table showing selective outputs corresponding to typical select signals of multiplexers in the multiplexer circuitry in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will hereinafter be described on the assumption that the afore-mentioned integer N is four.

As shown in FIG. 3, the first test head 11A carries four memory devices under test A1 to A4 and the second test head 11B carries four memory devices under test B1 to B4. A test pattern composed of data and an address is applied from the pattern generator 15 to the memory devices A1 to A4 and B1 to B4, wherein the data is written at the address. The data thus written in the memory devices under test is read out therefrom and provided to logical comparators 16₁ to 16₄ and 17₁ to 17₄ for logical comparison with expected values for each pin. The results of such logical comparisons are stored in fail memories. In this embodiment the memory devices Al to A4 and B1 to B4 each have four output pins and the results of logical comparisons A1₁ to A1₄, A2₁ to A2₄, A3₁ to A3₄, A4₁ to A4₄ and B1₁ to B1₄, B2₁ to B2₄, B3₁ to B3₄, B4₁ to B4₄ are selectively stored in the fail memories as described below.

FIG. 4 illustrates an example of a multiplexer circuit constructed so that the results of logical comparisons output from the logical comparators 16₁ to 16₄ and 17₁ to 17₄ in FIG. 3 can selectively be stored in fail memories 25₁ to 25₄ as in the prior art and that, if necessary, the logical sums (i.e. logical OR's) of the results of logical comparisons corresponding to the plurality of pins of the memory devices can selectively stored in the fail memories 25₁ to 25₄ according to the present invention.

In FIG. 4, first multiplexers 18₁ to 18₄ are supplied with the results of logical comparisons A1₁, A2₁, A3₁ and A4₁ corresponding to first pins of the memory devices A1 to A4 loaded on the first test head 11A depicted in FIG. 3, respectively. The results of logical comparisons A1₁, A1₂, A1₃ and A1₄ corresponding to the four output pins of the memory device A1 are ORed by a first OR circuit 27₁ and the resulting logical sum ORAl is also provided to the multiplexers 18₁ to 18₄ Based on select signals al, bl and cl applied thereto, the first multiplexer 18₁ selects and outputs any one of the four results of logical comparisons A1₁ to A4₁ and the logical sum ORAl. The first multiplexers 18₂, 18₃ and 18₄ each select and output any one of the four results of logical comparisons A1₁ to A4₁ and the logical sum ORAl, based on the select signals (a2, b2, c2), (a3, b3, c3) and (a4, b4, c4), respectively. Similarly, first multiplexers 18₅ to 18₈ are each supplied with the results of logical comparisons A1₂, A2₂, A3₂ and A4₂ corresponding to second output pins of the memory devices A1 to A4 and a logical sum ORA2 of the results of logical comparisons corresponding to the four output pins of the memory device A2 and provided from a first OR circuit 27₂, and one of the five inputs to each of the first multiplexers 18₅ to 18₈ is selected and output therefrom in accordance with the select signals (a1, b1, c1), (a2, b2, c2), (a3, b3, c3) and (a4, b4, c4), respectively. First multiplexers 18₉ to 18₁₂ are each supplied with the results of logical comparisons A1₃, A2₃, A3₃ and A4₃ corresponding to third output pins of the memory devices A1 to A4 and a logical sum ORA3 of the results of logical comparisons corresponding to the four output pins of the memory device A3 and provided from a first OR circuit 27₃, and one of the five inputs to each of the first multiplexers 18₉ to 18₁₂ is selected and output. First multiplexers 18₁₃ to 18₁₆ are each supplied with the results of logical comparisons A1₄, A2₄, A3₄ and A4₄ corresponding to fourth output pins of the memory devices A1 to A4 and a logical sum ORA4 of the results of logical comparisons corresponding to the four output pins of the memory device A4 provided from a first OR circuit 27₄, and the first multiplexers 18₁₃ to 18₁₆ each selectively output one of such five inputs thereto in the same manner as referred to above.

Likewise, four groups of second multiplexers 21₁ to 21₄, 21₅ to 21₈, 21₉ to 21₁₂ and 21₁₃ to 21₁₆ are supplied with the results of logical comparisons B1₁ to B4₁, B1₂ to B4₂, B1₃ to B4₃ and B1₄ to B4₄ for the corresponding output pins of the memory devices B1 to B4 loaded on the test head 11B and logical sums ORB1 to ORB4 of the results of logical comparisons corresponding to the four output pins of each of the memory devices B1 to B4 provided from second OR circuits 28₁ to 28₄, respectively. The second multiplexers each selectively output one of such five inputs thereto.

A third multiplexer 23₁ is supplied with the selected outputs from the first multiplexer 18₁ associated with the first test head 11A and the second multiplexer 21₁ associated with the second test head 11B and selectively outputs one of the two inputs in accordance with the select signal S₁. A third multiplexer 23₂ is supplied with the selected outputs from the first and second multiplexers 18₂ and 21₂ and selectively outputs either one of the two inputs in accordance with the select signal S₂. A third multiplexer 23₃ is supplied with the selected outputs from the first and second multiplexers 18₃ and 21₃ and selectively outputs either one of the two inputs in accordance with a select signal S₃. A third multiplexer 23₄ is supplied with the selected outputs from the first and second multiplexers 18₄ and 21₄ and selectively outputs either one of the two inputs in accordance with a select signal S₄. Similarly, each of third multiplexers 23₅ to 23₈, 23₉ to 23₁₂ and 23₁₃ to 23₁₆ is also supplied with the Selected outputs from the first and second multiplexers associated with the first and second test heads 11A and 11B, respectively, and selectively outputs either one of the two inputs in accordance with corresponding one of select signals S₁ to S₄.

The selected outputs from the third multiplexers 23₁ to 23₄ are provided as first bit inputs to fail memories 25₁ to 25₄ ; the selected outputs from the third multiplexers 23₅ to 23₈ are provided as second bit inputs to the fail memories 25₁ to 25₄ ; the selected outputs from the third multiplexers 23₉ to 23₁₂ are provided as third bit inputs to the fail memories 25₁ to 25₄ ; and the selected outputs from the third multiplexers 23₁₂ to 23₁₆ are provided as fourth bit inputs to the fail memories 25₁ to 25₄.

FIGS. 5 and 6 show concrete circuits interconnecting the OR circuits 27₁ and 28₁, the first multiplexers 18₁ to 18₄, the second multiplexers 21₁ to 21₄, the third multiplexers 23₁ to 23₄ and the fail memories 25₁ to 25₄ in FIG. 4. For convenience of description there are shown also in FIG. 6 the same OR circuits 27₁ and 28₁ as those depicted in FIG. 5.

As shown in FIGS. 5 and 6, the results of logical comparisons A1₁ to A4₁ corresponding to the first output pins of the memory devices 12₁ to 12₄ carried by the first test head 11A are input into the first multiplexers 18₁, 18₂, 18₃ and 18₄, respectively. The results of logical comparisons A1₁ to A1₄ corresponding to the first to fourth output pins of the first memory device 12₁ mounted on the first test head 11A are ORed by the first OR circuit 27₁ and its output ORAl is applied to each of the first multiplexers 18₁ to 18₄. The first multiplexer 18₁ responds to the select signals a1 and b1 to select and output any one of the inputs A1₁ to A4₁ and, when the select signal cl is high, outputs the output ORAl from the first OR circuit 27₁. The first multiplexer 18₂ responds to the select signals a2 and b2 to select and output any one of the inputs A1₁ to A4₁ and, when the select signal c2 is high, outputs the output ORAl from the first OR circuit 27₁. The first multiplexer 18₃ responds to the select signals a3 and b3 to select and output any one of the inputs A1₁ to A4₁ and, when the select signal c3 is high, outputs the output ORAl from the first OR circuit 27₁. The first multiplexer 18₄ responds to the select signals a4 and b4 to select and output any one of the inputs A1₁ to A4₁ and, when the select signal c4 is high, outputs the output ORAl from the first OR circuit 27₁.

The results of logical comparisons B1₁ to B4₁ corresponding to the first output pins of the memory devices 14₁ to 14₄ carried by the second test head 11B are input into the second multiplexers 21₁, 21₂, 21₃ and 21₄, respectively. The results of logical comparisons corresponding to the first to fourth output pins of the memory device 14₁ mounted on the second test head first memory device 14₁ mounted on the second test head 11B are ORed by the second OR circuit 28₁ and its output ORB1 is applied to each of the second multiplexers 21₁ to 21₄. As is the case with the first multiplexers 18₁ to 18₄, the second multiplexers 21₁ to 21₄ each selectively output any one of the five inputs in accordance With the select signals (a1, b1, c1), . . . (a4, b4, c4).

The outputs of the first and second multiplexers 18₁ and 21₁ are input into the third multiplexer 23₁ and either one of the two inputs is selectively output therefrom in accordance with the select signal S₁ ; the outputs of the first and second multiplexers 18₂ and 21₂ are input into the third multiplexer 23₂ and either one of the two inputs is selectively output therefrom in accordance with the select signal S₂ ; the outputs of the first and second multiplexers 18₃ and 21₃ are input into the third multiplexer 23₃ and either one of the two inputs is selectively output therefrom in accordance with the select signal S₃ ; and the outputs of the first and second multiplexers 18₄ and 21₄ are input into the third multiplexer 23₄ and either one of the two inputs is selectively output therefrom in accordance with the select signal S₄. The outputs of the third multiplexers 23₁ to 23₄ are stored in the fail memories 25₁ and 25₄, respectively.

The connections of the first multiplexers 18₅ to 18₈, the second multiplexers 21₅ to 21₈ and the third multiplexers 23₅ to 23₈ in FIG. 4 are similar to those shown in FIGS. 5 and 6. Further, the first multiplexers 18₉ to 18₁₂, the second multiplexers 21₉ to 21₁₂, the third multiplexers 23₉ to 23₁₂ and the first multiplexers 18₁₃ to 18₁₆, the Second multiplexers 21₁₃ to 21₁₆, the third multiplexers 23₁₃ to 23₁₆ are also connected in a manner Similar to those shown in FIGS. 5 and 6. Hence, these circuit connections are not shown, for the sake of brevity.

Table I in FIG. 7 shows, by way of example, the selected outputs of the first and second multiplexers 18₁ to 18₄ and 21₁ to 21₄ and the selection of the third multiplexers 23₁ to 23₄ in response to the application thereto of the select signals identified by a, b, c and S. A horizontal column labeled "1st MPX" shows the five inputs A1₁ to A4₁ and ORAl to the first multiplexers 18₁ to 18₄ associated with the first test head 11A and selected outputs corresponding to combinations of the select signals (a, b, c). Another horizontal column labeled "2nd MPX" shows the five inputs B1₁ to B4₁ and ORB1 to the second multiplexers 21₁ to 21₄ associated with the second test head 11B and selected outputs corresponding to combinations of the select signals (a, b, c). A vertical column labeled "3rd MPX" shows the multiplexer outputs which the third multiplexers 23₁ to 23₄ select in accordance with the select signal S.

Also in connection with the groups of first multiplexers 18₅ to 18₈, 18₉ to 18₁₂, 18₁₃ to 18₁₆, the groups of second multiplexers 21₅ to 21₈, 21₉ to 21₁₂, 21₁₃ to 21₁₆ and the groups of third multiplexers 23₅ to 23₈, 23₉ to 23₁₂, 23₁₃ to 23₁₆, the selected outputs based on the select Signals a, b, c and S can easily be determined in the same manner as described above in respect of Table I. Hence, such selected outputs are not shown.

Now, let it be assumed that the select signals are as follows: al=0, bl=0, cl=0, a2=1, b2=0, c2=0, a3=0, b3=1, c3=0, a4=1, b4=1, c4=0, S₁ =0, S2=0, S3=0, S4=0. In the first multiplexers 181, 182, 18₃ and 18₄ the outputs A1₁, A2₁, A3₁ and A4₁ are selected, respectively; in the first multiplexers 18₅, 18₆, 18₇ and 18₈ the outputs A1₂, A2₂, A3₂ and A4₂ are selected, respectively; in the first multiplexers 18₉, 18₁₀, 18₁₁ and 18₁₂ the outputs A1₃, A2₃, A3₃ and A4₃ are selected, respectively; and in the first multiplexers 18₁₃, 18₁₄, 18₁₅ and 18₁₆ the outputs A1₄, A2₄, A3₄ and A4₄ are selected, respectively. Furthermore, since the third multiplexers 23₁ to 23₁₆ all select the first test head 11 side (the first multiplexer outputs), the results of logical comparisons A1₁ to A1₄ of the memory device 12₁ are stored in the fail memory 25₁ ; the results of logical comparisons A2₁ to A2₄ of the memory device 12₂ are stored in the fail memory 25₂ ; the results of logical comparisons A3₁ to A3₄ of the memory device 12₃ are stored in the fail memory 25₃ ; and the results of logical comparisons A4₁ to A4₄ of the memory device 12₄ are stored in the fail memory 25₄. In other words, the results of logical comparisons of the memory devices 12₁ to 12₄ are stored individually in the fail memories 25₁ to 25₄.

In the case where the select signals (a1, b1, c1), ... (a4, b4, c4) for the second multiplexers 21₁ to 21₁₆ have the same values as in the above case and the select signals S₁, S₂, S₃ and S₄ are all set to 1, the third multiplexers 23₁ to 23₁₆ select the second test head 11B side (the second multiplexer outputs), and consequently, the results of logical comparisons B1₁ to B1₄ of the memory device 14₁, the results of logical comparisons B2₁ to B2₄ of the memory device 14₂, the results of logical comparisons B3₁ to B3₄ of the memory device 14₃ and the results of logical comparisons B4₁ to B4₄ of the memory device 14₄ are stored in the fail memories 25₁, 25₂, 25₃ and 25₄, respectively. That is, the results of logical comparisons of the memory devices 14₁ to 14₄ are stored individually in the fail memories 25₁ to 25₄.

Setting the select signals a1, b1, a2 and b2 to 0, a3, b3, a4 and b4 to arbitrary values, S₁ to 0, S₂ to 1, S₃ to 0, S₄ to 1, the select signal cl of each of the first multiplexers 18₁, 18₅, 18₉ and 18₁₃ to 1, the other select signals c2, c3 and c4 to 0, the select signal cl of each of the second multiplexers 21₁, 21₅, 21₉ and 21₁₃ to 1 and the other select signals c2, c3 and c4 to 0, the third multiplexer 23₁ provides the output ORAl of the first OR circuit 27₁ and, similarly, the third multiplexers 23₅, 23₉ and 23₁₃ provide the outputs ORA2, ORA3 and ORA4 of the first OR circuits 27₂, 27₃ and 27₄, respectively. The outputs of the third multiplexers 23₁, 23₅, 23₉ and 23₁₃ are stored in the fail memory 25₁. Furthermore, the third multiplexer 23₂ provides the output ORB1 of the second OR circuit 28₁, the third multiplexers 23₆, 23₁₀ and 23₁₄ provide the outputs ORB2, ORB3 and ORB4 of the second OR circuits 28₂, 28₃ and 28₄, respectively, and the outputs of the third multiplexers 23₂, 23₆, 23₁₀ and 23₁₄ are stored in the fail memory 25₂. In this way, the logical sums ORA1, ORA2, ORA3 and ORA4 of the results of logical comparisons A1₁ to A1₄, A2₁ to A2₄, A3₁ to A3₄ and A4₁ to A4₄ of the memory devices 12₁, 12₂, 12₃ and 12₄ loaded on the first test head 11A are stored in the fail memory 25₁. Likewise, the logical sums ORB1, ORB2, ORB3 and ORB4 of the results of logical comparisons B1₁ to B1₄, B2₁ to B2₄, B3₁ to B3₄ and B4₁ to B4₄ of the memory devices 14₁, 14₂, 14₃ and 14₄ loaded on the second test head 11B are stored in the fail memory 25₂. Thus, the results of logical comparisons of the memory devices on the first and second test heads 11A and 11B can be stored in the fail memories 25₁ and 25₂ concurrently. In this instance, only the OR of the results of logical comparisons corresponding to the first to fourth output pins of each memory device can be known, that is, the results of logical comparisons for each pin are not stored in the fail memory. However, in the case of a memory device in which a plural-bit word is accessed by one address, i.e. in a memory device of a 4-bit word configuration, it is necessary only to check the memory device for an error for each word (or address); there is no need of making a check for an error for each bit (or pin).

The above embodiment has been described to include two test heads, but the number of test heads is not limited specifically thereto but may be one or more than two. While the above embodiment has been described in connection with the case where four memory devices under test are loaded on each test head and each have four output pins, it can also be considered that the memory devices Al and A2 in FIG. 3, for example, are those into which one memory device having eight output pins has been provisionally divided for test. The same is true of the memory devices A3, A4, Bl, B2, B3 and B4. The number of fail memories need not always be four but needs only to be equal to at least the number of test heads used.

As described above, according to the present invention, the results of logical comparisons corresponding to a plurality of pins of each memory device are ORed and the OR is selectable. Hence, the results of logical comparisons corresponding to each pin of a plurality of memory device on each test head can be stored in separate fail memories as in the prior art, besides, by selecting the above-mentioned OR, the results of tests of the memory devices on first and second test heads can be stored in separate fail memories concurrently.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention. 

What is claimed is:
 1. A semiconductor memory tester comprising:test pattern generating means for applying a test pattern to each of a plurality of memory devices installed on a test head and each having a plurality of output pins; logical comparator means for logically comparing the output of each of said memory devices with an expected value for each output pin and for outputting the result of the logical comparison; a plurality of OR circuits each of which obtains the OR of said results of logical comparisons corresponding to a predetermined plurality of pins of said each memory device; a plurality of multiplexer means each of which receives a plurality of said results of logical comparisons for corresponding pins of said memory devices and the OR output of one of said plurality of OR circuits and selectively outputs a desired one of said inputs; and fail memory means for storing the outputs of said plurality of multiplexer means.
 2. The semiconductor memory tester of claim 1, which further includes: second logical comparator means whereby the output of each of a plurality of second semiconductor memory devices installed on another test head is subjected to a logical comparison with an expected value for each pin and outputs the result of logical comparison; a plurality of second OR circuits each of which obtains the OR of said results of logical comparisons corresponding to a predetermined plurality of pins of said each second memory device; a plurality of second multiplexer means each of which receives a plurality of said results of logical comparisons for corresponding pins of said second memory devices and the OR output of one of said plurality of second OR circuits and selectively outputs a desired one of said inputs; and a plurality of third multiplexer means for selectively outputting the outputs of said first and second multiplexer means; and wherein said fail memory means includes at least two fail memories which are supplied with the outputs of said plurality of third multiplexer means.
 3. A semiconductor memory tester comprising:pattern generating means for applying a test pattern to N first memory devices installed on a first test head and each having P output pins, both N and P being integers equal to or greater than 2; first logical comparator means for logically comparing the output of each of said first memory devices with an expected value for each output pin and for outputting the result of said logical comparison; second logical comparator means whereby the output of each of N second memory devices installed on a second test head and supplied with said test pattern is logically compared with an expected value for each output pin and from which the result of said logical comparison is output; N first OR circuits each of which receives the results of said logical comparisons from said first logical comparator means and obtains the OR of said results of logical comparisons for a predetermined plurality K of pins of each of said memory devices, K being an integer in the range of 2≦K≦P; N second OR circuits each of which receives the results of said logical comparisons from said second logical comparator means and obtains the OR of said results of logical comparisons for a predetermined K pins of each of said memory devices; N first multiplexers provided in association with each of corresponding pins of each of said N first memory devices, each of said N first multiplexers receiving N results of said logical comparisons for the corresponding pins of said N first memory devices and the OR output of one of said N first OR circuits and selectively outputting a desired one of said inputs; N second multiplexers provided in association with each of corresponding pins of each of said N second memory devices, each of said N second multiplexers receiving N results of said logical comparisons for the corresponding pins of said N second memory devices and the OR output of one of said N second OR circuits and selectively outputting a desired one of said inputs; a plurality of third multiplexers each of which is supplied with at least two corresponding outputs of said first and second multiplexers and selectively outputs either one of said inputs; and at least two fail memories to which the outputs of said third multiplexers are distributed. 